Water-containing substances, such as kitchen garbage, residue by food manufacturing, an effluent of Shochu production, an effluent of palm oil production, livestock excreta, sludge generated by sewage treatment and the like, have high water content. Thus, it is difficult to directly incinerate them. Heretofore, many studies and proposals have been performed about the use of these water-containing substances by reducing the volume of them (i.e., drying them to reduce their volume) and then using them as an organic waste fuel as they are or after mixing with oil, and about the recycling of the water-containing substances as livestock feed or fertilizer by applying fermentation techniques.
Patent literature 1 discloses a method for treating food residue characterized by adding to the food residue a dispersant such as a surfactant, an antifoamer, a flocculant, a fat and oil, or the like, mixing, and drying by heat. The treated object thus obtained is, for example, in a powdery or granular state, and is used as feed or diet, or as a raw material for producing fertilizer.
Patent literature 2 discloses a method for producing an organic waste fuel which comprises steps of performing methane fermentation of an organic waste such as food waste or the like, mixing the residue of the methane fermentation thus obtained with waste oil, and heating the obtained mixture under condition of a reduced pressure at a constant temperature (70 to 100 degrees Celsius). The water content of the organic waste fuel obtained by this method is from 20 to 40%.
Patent literature 3 discloses a method for producing feed which comprises steps of adding Koji-kin (fungi belong to Aspergillus) or rice malt, and an oil and fat to an organic waste such as kitchen garbage or the like, and fermenting the organic waste with the Koji-kin to remove water in the organic waste by evaporation.
Patent literature 4 discloses a method for treating biomass, which comprises fermenting biomass such as dehydrated cake of sludge generated by sewage treatment, or the like, to produce an acid, dehydrating the obtained biomass in oil under certain conditions of temperature and pressure (for example, at a temperature of about 150 degrees Celsius under a pressurized condition of about 0.3 MPa), under which conditions the acid evaporates, to obtain a mixture of the oil with a dry biomass and vapor containing the acid, deoiling the mixture to obtain the dry biomass, and condensing the vapor containing the acid to obtain an acid-containing water.
Patent literature 5 discloses a device of circulation type for dry-treatment at ordinary temperature employing a repeating system, one cycle of which comprises heating organic substances such as food waste or the like at a temperature within an ordinary range in a treatment tank while agitating the organic substances, having air holding water that is contained in the organic substances as water vapor to bring the water vapor out of the treatment tank, condensing the water vapor by a condenser that is separately arranged to remove water, and returning the air, from which water has been removed, to the treatment tank. Although the drying method by using this device does not use an oil and fat, patent literature 5 discloses that the drying temperature may be several degrees Celsius higher than that in the treatment tank, and that heating may be performed by a heater to be about 37 degrees Celsius if the temperature in the treatment tank is, e.g., about 30 degrees Celsius in a summer season.